You've learned that master documents can simplify collaboration. You can delegate parts of your document for others to edit while working on other elements yourself; then when you're ready, you can review and edit the document as a whole.
By default, Word gives you complete access to any subdocument of which you are the author (assuming that the subdocument is stored in a folder you have rights to access). Word gives you more limited access to subdocuments you did not author.
Word determines who the author is by looking in the Author field of the File, Properties dialog box. How does the name of the author get there in the first place? It comes from the information you gave Word when you installed ...